Valve assembly

ABSTRACT

A valve assembly for load pressure-independent control of consumers is disclosed, which includes a main valve having a directional element and a metering throttle part. The valve assembly includes a shut-off valve for leakage-free shutting off of a work line leading to the consumer. This shut-off valve is designed with a pilot valve whereby a pressure chamber of the shut-off valve acting in the closing direction may be relieved towards a tank or a low-pressure source. The pilot valve is, in accordance with the invention, controlled open by a main slide of the main valve.

The invention relates to a valve assembly for load pressure-independentcontrol of consumers.

BACKGROUND OF THE INVENTION

The like valve assemblies are employed, e.g., in LIFD (Load-independentFlow Distribution) systems as known from WO 95/32364 A1 and Data SheetRD 64 127/04.98 (Hydraulic Valves for Mobile Applications). An LIFDsystem includes, for instance, a variable displacement pump which may beregulated such as to generate at its output a pressure that is higherthan the highest load pressure of all hydraulic consumers by a specificdifferential amount.

Each consumer is associated with a variable metering throttle with adownstream pressure compensator, with the latter maintaining thepressure drop across the metering throttle constant, so that the amountof pressure medium flowing to the corresponding hydraulic consumerdepends not on the load pressure of the consumer or on the pumppressure, but solely on the open cross-section of the metering throttle.With the aid of the pressure compensators of the system it is achievedthat in a case where the hydraulic pump was adjusted to the maximumstroke volume and the pressure medium flow is not sufficient formaintaining the specified pressure drop across the metering throttles,the pressure compensators of all of the actuated hydraulic consumers areadjusted in the closing direction, so that all the pressure medium flowsare reduced by a same percentage. Due to this load-pressure independentflow distribution (LIFD), all of the actuated consumers move at avelocity that is reduced percentually by a same value.

In the solution in accordance with WO 95/32364 A1, if a consumer issupported for a prolonged period of time, it may happen that the latterwill subside due to a small leakage flow between the work port subjectedto pressure medium and the pressure medium tank of the system.

In order to avoid such a leakage flow, it is suggested in DE 196 46 447A1 to insert in the respective work port a check valve that is capableof being controlled open and permits to shut off the work line leadingto the consumer and subjected to pressure medium without any leakage. Itis a drawback in this solution that considerable structural space isused up by mounting the check valve onto the work port.

SUMMARY

Instead of the add-on check valve it is also possible to realize thevalve housing designed in disc design with an integrated check valve. Assuch leakage-free designs, other than the above-described conventionaldesigns, are only utilized comparatively rarely, such a special designmight only be realized at a comparatively high financial expense.

In comparison, the invention is based on the objective of furnishing acompact valve assembly that may be manufactured at minimum expense.

This objective is attained through a valve assembly having the featuresof claim 1.

In the valve assembly in accordance with the invention, the shut-offvalve permitting leakage-free shutting is designed with a pilot valvethrough which the shut-off valve may be controlled open so as to permita pressure medium flow towards the tank. In accordance with theinvention, the pilot valve is actuated by an actuation movement of amain slide of a main valve whereby the work ports of the valve assemblythat are connected with the consumer may be connected with a pressuremedium tank or with a pressure passage conducting the pump pressure. Inaccordance with the invention, the pilot valve is arranged substantiallycoaxially with the main slide of the main valve determining thedirection of pressure medium flow, and is in operative connection withthe latter, so that the valve assembly may be given an extremely compactdesign. The solution of the invention including the pilot valve that isdirectly actuated through the main valve allows to laterally mount ontoa standard disc a valve housing, wherein the pilot valve, the associatedshut-off valve, and the associated work port are formed.

In accordance with the invention it is preferred if a tappet is formedat the main slide on the end face side of the main valve, which tappetraises the pilot piston from its pilot control seat upon correspondingdriving of the main slide, so that a control surface of the shut-offvalve that acts in the closing direction is relieved of load.

The main slide has a particularly simple construction if the tappet isinserted into the main slide as an insert member.

The laterally added-on valve housing may be given a particularly simpleconstruction if the axis of the pilot valve extends at a paralleldistance from the axis of the shut-off valve.

In a solution having a particularly simple construction, the controlsurface of the shut-off valve acting in the closing direction isconnected via the pilot valve with a control port conducting, in theevent of the axial displacement of the main slide of the main valvedescribed at the outset, a comparatively low pressure corresponding,e.g., to the tank pressure.

In order to prevent abrupt opening of the shut-off valve, the pilotpiston of the pilot valve is preferably provided with fine-controlgrooves through which the connection with the low-pressure port, forexample the above described control port, is opened gradually.

In order to avoid pressure peaks and cavitation phenomena, a pressurelimiting/anti-cavitation valve is associated to the shut-off valve. Aport of this valve is connected with a tank port of the valve assemblyvia a tank passage. The construction of the laterally added-on valvehousing is particularly simple if this tank passage encompasses ahousing cartridge of the shut-off valve as an annular passage thatpreferably extends from the laterally added-on valve housing into thevalve disc.

The main slide is biased through reset spring means into its neutralposition. These reset spring means are configured such that the resetspring acting against the direction of actuation for actuating the pilotvalve will only take effect after a predetermined initial stroke of themain slide, so that initially, the main slide essentially only has to bedisplaced against the force acting on the pilot piston.

In a preferred embodiment, this is achieved in that the respective resetspring is supported on a spring cup which contacts a support shoulderonly after the above mentioned initial stroke, so that followingcontact, further axial displacement of the main slide takes placeagainst the force of the reset spring.

In a preferred embodiment, the main valve forms a variable meteringthrottle followed downstream by a pressure compensator that is shared bythe two work ports and the axis of which is preferably capable of beingaccommodated in the valve disc perpendicularly to the axis of the mainvalve.

Other advantageous developments of the invention are subject matter ofthe further subclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following a preferred embodiment of the invention shall beexplained in more detail with the aid of schematic drawings, wherein:

FIG. 1 shows a highly simplified circuit diagram of the valve assemblyof the invention;

FIG. 2 shows a sectional view of the valve assembly of the invention indisc design; and

FIG. 3 shows a partial representation of the valve assembly of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The valve assemblies described hereinbelow are used for driving mobilework tools, such as shovel diggers, and customarily are designed in discdesign. Here several ones of the valve discs are combined into onecontrol block, with the valve elements for each function (actuateshovel, travelling mechanism, lift/lower) are respectively combined inone valve disc.

FIG. 1 shows a highly simplified hydraulic circuit diagram of adirectional control valve element 2 for driving a double-actionhydraulic cylinder 4.

The directional control valve element 2 is designed with a pressure portP, a tank port T, two work ports A, B, an LS port (not represented), andtwo control ports a, b. The work port A is connected with a pistonbase-side cylinder chamber 6, and the other work port B with an annularchamber 8 of the hydraulic cylinder. The directional control valveelement 2 includes a main valve 10 designed as a continuously variabledirectional control valve, the main slide 12 of which is biased by resetspring means 14 into the represented Zero position. The displacement ofthe main slide 12 is achieved through application of a control pressurevia the two control ports a, b. As shall be set forth in more detailhereinbelow, the main valve 10 is designed with a velocity element (notrepresented in FIG. 1) constituting a metering throttle, and adirectional element determining the pressure medium flow from and to thechambers 6, 8, respectively. Downstream from the metering throttle apressure compensator 16 (indicated schematically in FIG. 1) is arranged,which is acted upon by the pressure at the outlet of the meteringthrottle in the opening direction, and by the load pressure at theconsumer (hydraulic cylinder 4) in the closing direction. As wasmentioned at the outset, the downstream pressure compensator 16maintains constant the pressure drop across the metering throttlerepresented by the main valve 10, whereby a load pressure independentflow distribution (LIFD) is made possible. Such LIFD controls are knownfrom the prior art, e.g., WO 95/32364 A1, so that further explanationsmay be omitted.

For the case that the hydraulic cylinder 4 it to be held in an extendedposition, the pressure medium must be held under pressure in thecylinder chamber 6 without any leakages. To this end there is providedin the directional control valve element 2 a pilot-controlled shut-offvalve 18, the pilot valve 20 of which is arranged about coaxially withthe valve axis of the main valve 10. In other words, shut-off valve 18and pilot valve 20 are arranged separately from each other in thevariant in accordance with the invention. The shut-off valve 18 permitsa pressure medium flow from the main valve 10 towards the work port A; apressure medium flow in the reverse direction is only possible when thepilot valve 20 is actuated. Via the pilot valve 20 it is possible torelieve pressure on a control surface that acts in the closing directionof the shut-off valve, so that the shut-off valve 18 is opened by thepressure in the cylinder chamber 6, and pressure medium flow from thecylinder chamber 6 towards the tank port T is enabled. Actuation of thepilot valve 20 is achieved in accordance with the invention through theintermediary of a tappet 22 of the main slide 12, whereby a pilot piston24 may be raised from a valve seat, so that the pressure medium actingon the control surface of the shut-off valve 18 in the closing directionmay be relieved towards the control port a. The latter conducts acontrol pressure approximately corresponding to the tank pressure in theevent of a displacement of the main slide 12 towards the pilot valve 20.

In a work passage 26 between the work port A and the main valve 10 apressure/anti-cavitation valve 28 is arranged, which on the one handlimits the pressure in the work passage 26 to a maximum value, opens upa tank passage 30 leading to the tank port T when the pressure isexceeded, and enables replenishing of pressure medium from the tankpassage 30 in the event of a very rapid extension movement of thehydraulic cylinder 4 caused, e.g., by travelling movements etc.

In another work passage 32 leading to the work port B a pilot-controlledpressure/feed valve 34 is provided, whereby the pressure in the anotherwork passage 32 may be limited to a variable maximum value and may befed via the pressure medium from the tank passage 30 into the anotherwork passage 32.

By combining several ones of the above described directional controlvalve elements 2 including an inlet element and a terminal plate, it ispossible to compose a highly compact control block for fulfilling mobilehydraulics tasks which may, thanks to the sandwich design, very easilybe adapted to different operating conditions.

FIG. 2 is a sectional view of a directional control valve element 2 inaccordance with FIG. 1. Such a directional control valve elementrealized in disc design has a valve disc 36 which essentially is astandard component as described, e.g., in the applicant's Data Sheet RD64 127/04.98. Laterally on the valve disc 36 a valve housing 38 is addedon, wherein those valve elements are combined that are not present in astandard design in accordance with the above identified data sheet.

The work port B, the pressure port P, the tank port T, the control portb (not represented), and the LS port are formed in the valve disc 36.The valve disc 36 is penetrated by a valve bore 40 wherein the mainslide 12 of the main valve 10 is guided. A central pressure chamber 42,a pressure compensator passage 44, two connecting passages 46, 48, thework passage 26, the another work passage 32, and tank passage sections50, 52 merge radially into the valve bore 40. The main slide 12 isformed with an end face-side annular collar 54 at its left-hand endsection and with a annular collar 56 at its right-hand end section. Inaddition the main slide 12 has two control collars 58, 60 of thedirectional element, with the control collar 58 being associated to workport A, and the control collar 60 to the work port B. Between the twocontrol collars 58, 60 there is a measuring orifice collar 62 providedin both annular end faces thereof with fine-control grooves 64.

In the represented basic position of the main slide 12, the endface-side annular collars 54, 56 close the tank passage sections 50, 52,the control collars 58, 60 close the two connecting passages 46, 48, andthe fine-control grooves 64 of the measuring orifice collar 62 areblocked by the central land of the valve bore 40.

On the right-hand lateral face of valve disc 36 in the view of FIG. 2,the reset spring means 14 is added on. It comprises a reset spring 68,the right-hand end section of which in the view of FIG. 2 is supportedon a spring cup 70 that is guided, in an axially slidable manner, on aslideway 72 inserted end face-wise in the main slide 12. This slidewayhas a radial shoulder 74 against which the spring cup 70 is biased tothe right through the force of the reset spring 68.

The other end of the reset spring 68 is supported on another spring cup76 which in turn is biased by the reset spring 68 against the end faceof the main slide 12. In this basic position a play S exists between theleft-hand end face 78 of the spring cup in the view of FIG. 2 and acontact shoulder 80. As may be seen in FIG. 2, the another spring cup 76is slidingly guided on the right-hand annular collar 56 of the mainslide 12 and on the slideway 72. The spring cups 70, 76 may thusapproach each other in the event of an axial displacement of the mainslide 12. The spring chamber 82 receiving the reset spring 68 and thespring cup 70, 76 is encompassed by a cover 84 and may be subjected tothe pressure at control port b.

As was explained by referring to FIG. 1, the work passage 32 associatedto the work port B may be connected with the tank passage T via thepressure/feed valve 34 in order to limit the pressure in the workpassage 26 or replenish pressure medium from the pressure medium tank.The pressure/feed valve represented in FIG. 2 is designed with pilotcontrol. It thus is a standard component realized in cartridge design,with further explanations accordingly being omitted.

Into the pressure compensator passage 44 the pressure compensator 16 isinserted, the valve axis of which extends perpendicularly to that of themain valve 10. The pressure compensator piston 82 is subjected in theopening direction to the pressure downstream from the measuring orifice82 defined by the measuring orifice collar 62, and in the closingdirection to the pressure at the LS passage as well as the force of acontrol spring.

The outlet port 84 of the pressure compensator 16 is connected viarespective load-holding valves 86, 88 with the connecting passage 46associated with the work port A and with the connecting passage 48associated with the work port B, respectively. Such a load-holding valve86 fundamentally constitutes a check valve permitting a pressure mediumflow from the outlet port 84 of the pressure compensator 16 to theassociated passage 44 or 46 while blocking in the opposite direction.

In the embodiment represented in FIG. 2, the work port A provided at thestandard valve disc 36 is shut off by a closure member 90. The work portA is formed in the laterally added-on valve housing 38. In this valvehousing 38 there are accommodated the pilot valve 20 formed coaxiallywith the main valve 10, the shut-off valve 18 associated thereto, andthe pressure/anti-cavitation valve 28 associated to the work port A,with the axes of these valve elements extending in parallel (18, 28) orcoaxially (20), respectively, with the axis of the main valve 10.

Details of the valve housing shall hereinbelow be explained by referringto the partial representation in accordance with FIG. 3.

The shut-off valve 18 includes a housing cartridge 92 inserted in atransverse bore 96, which penetrates the valve housing 38 in thehorizontal direction (view of FIG. 3), and the end section 94 of whichplunges into an extension of the transverse bore 96 that communicateswith the work passage 26 in the valve disc 36.

The housing cartridge 92 has a stepped axial bore 98 on which a valveseat 100 is formed, against which a cone 102 of the shut-off valve 18 isbiased through the intermediary of a compression spring 104. The latteris supported on a closure screw 106 screwed into the housing cartridge92.

A group of radially arranged bores 108 of the housing cartridge 92,which in turn is connected with a passage 110 leading to the work portA, merges into the axial bore 98. In the range of the radially arrangedbores 108 there is formed, in the jacket of the cone 102, a connectingbore 109 whereby a spring chamber 105 accommodating the compressionspring 104 is connected with the passage 110, so that the shut-off valveis biased against the valve seat 100 by the pressure in the passage 110in addition to the force of the compression spring 104.

In the valve housing 38 there is moreover formed a tank bore 112realized as an angular bore, the horizontal leg of which (view of FIG.3) does not intersect the passage 110. This may be achieved, forinstance, by the tank bore 112 being offset relative to the passage 110.In addition both the work passage and also the tank passage may beformed by parallel bores.

In the represented embodiment, the tank bore 112 is formed by a verticalbore and a horizontal bore intersecting the latter, wherein the verticalbore is closed by a closure member 114, while thepressure/anti-cavitation valve 28 is inserted in the horizontal bore. Inaccordance with FIG. 3, the tank bore 112 opens into an annular chamber116 delimited by a radially expanded part of the transverse bore 96 ofthe valve housing 38 and the outer periphery of the end section 94 ofthe housing cartridge 92. The tank passage section 50 merges into thisannular chamber 116, so that the tank bore 112 is connected with thetank port T.

In accordance with FIG. 3, the pressure/anti-cavitation valve 28 has ananti-cavitation piston 118 that is biased against a seat 122 by a spring120. Inside the anti-cavitation piston 118 a pressure limiting piston124 is guided which is biased against a valve seat (not represented) inthe anti-cavitation piston 118 through the intermediary of a pressurelimiting spring 126. In the represented embodiment, the pressurelimiting spring 126 attacks on a cup 128 formed on a part of thepressure limiting piston 124 that protrudes into the spring chamber.

In the valve housing 38 there is moreover formed a control passage 130merging on the one hand into a radial bore 132 of the housing cartridge92 and on the other hand into a stepped pilot bore 134 accommodating thepilot valve 20. In this stepped pilot bore 134, the pilot piston 24 isbiased against a pilot control seat 138 through the intermediary of apilot spring 136, with the pilot spring 136 being supported on a closuremember 140. Via this valve seat 138 the pilot bore 134 merges into acontrol chamber 142 into which the left-hand annular collar 54 of themain slide 12 merges, and which is connected with the control port a.Into the end face of the annular collar 54 an insert member 144 carryingthe tappet 22 is inserted, with the tappet 22 being shaped such that itis capable of plunging into the pilot control seat 138 in order to raisethe pilot piston 24 from the pilot control seat 138 against the force ofthe pilot spring 136. The pilot piston 24 is designed as a steppedpiston, with the cone of the pilot piston 24 that co-operates with thepilot control seat 138 has a substantially smaller diameter than theadjacent annular end face 135 or the rear-side end face 137 of the pilotpiston 24. On the outer periphery of the pilot piston 24, fine-controlnotches 139 are formed whereby a pressure chamber delimited by theannular end face 135 and the pilot control seat 138 is connected withthe control passage 130. The pressure present in this pressure chamberis tapped via a compensation bore 152 penetrating the pilot piston 24and signalled into the spring chamber 136, so that the end faces of thepilot piston 24 are substantially pressure-balanced.

The portion of the control passage 130 merging into the pilot bore 134is in turn formed by a bore that is closed with the aid of a closuremember 146.

For extending the hydraulic cylinder 4, a control pressure is applied tothe control port a, while the control port b is connected with the tankor with another low-pressure source. Owing to the pressure differenceacting on the end faces, the main slide 12 is displaced from itsspring-biased basic position (FIG. 2) to the right. The connectionbetween the pressure chamber 42 and the pressure compensator passage 44is controlled open via the fine-control grooves 64—the metering throttle82 is opened, and pressure medium may flow to the inlet port of thepressure compensator 16. As was set forth at the outset, the latteradjusts itself, as a function of the acting load pressure and of thepressure downstream of the measuring orifice 82, in a control positionwherein the pressure drop across the measuring orifice 82 may bemaintained constant independently of load pressure. The pressure mediumflows from the pressure compensator 16 via the load-holding valve 86into the connecting passage 46. Due to the axial displacement of themain slide 12, a control land formed by the control collar 58 hascontrolled the connection between the connecting passage 46 and theanother work passage 32 open, so that the pressure medium may flow invia the work passage 26 into the axial bore 98 of the housing cartridge92 of the shut-off valve 18. The pressure of the pressure medium is sohigh that the cone 102 is raised from the valve seat 100 against theforce of the compression spring 104 and the low pressure force in thespring chamber 105, and the pressure medium may flow in via the radiallyarranged bores 108 and the passage 110 to the work port A and from therevia a work line 148 (FIG. 1) into the cylinder chamber 6—the piston ofthe hydraulic cylinder 4 extends. The pressure medium displaced from theannular chamber 8 flows via the line 150 (FIG. 1) to the pressure port Band from there into the work passage 32. Owing to the axial displacementof the main slide 12, the right-hand, end face-side annular collar 56has controlled open the connection between the work passage 26 and thetank passage section 52 by means of a control land, so that the pressuremedium may flow off from the work passage 32 towards the tank port T.

When a maximum pressure that is adjusted via thepressure/anti-cavitation valve 28 is exceeded, the smaller pressurelimiting piston 124 is raised from its seat in the anti-cavitationpiston 118 against the force of the pressure limiting spring 126 (to theright), so that the connection with the tank bore 112 is opened, andpressure medium may flow off via the tank bore 112, the annular chamber116, and the tank passage section 50 towards the tank port T until thepressure at work port A has dropped below the maximum adjusted pressure.

The pressure/feed valve 34 is designed in a known manner to have ananti-cavitation function, so that pressure medium may be fed from thetank passage section 52 in the work passage 26 in order to avoidcavitations.

When the hydraulic cylinder 4 extends, the pilot valve 20 is closed asit is biased against the pilot control seat 138 by the force of thepilot spring 136. The pressure forces acting on the pilot piston 24 areessentially balanced, for the valve seat has a substantially smallercross-section in comparison with the end faces of the pilot piston, andthe end face portions of the pilot piston (with the exception of thearea of the pilot control seat 138) are subjected to a same pressure.

Following the initial stroke S, the spring cup 76 contacts the contactshoulder 80, so that the further axial displacement of the main slide 12is only possible against the force of the reset spring 68 received witha bias. Having passed through this initial stroke S, the pilot piston 24is opened to such an extent as to be perfectly pressure-balanced, sothat the force acting from the side of the pilot valve 20 on the mainslide 12 is negligible, and its axial displacement is thus furthermoreonly influenced by the pressure at the control port b and the force ofthe reset spring 68 (equally about 5 bar). As a result of this initialstroke S, the main slide is thus not abruptly moved to the left when thepilot valve is opened and the pressure force (5 bar) correspondinglyceases, so that continuous control is ensured.

The fine-control notches 139 ensure that the shut-off valve 18 will onlybe relieved gradually, so that abrupt lowering of the load may beprevented.

For the case that the pressure in the passage 110 drops below the tankpressure (cavitation risk), the anti-cavitation piston 118 rises fromits seat 122 against the force of the spring 120, so that pressuremedium may be fed via the tank bore 112 into the passage 110.

In order to hold the load, the main valve 10 is moved back into itsneutral position, so that the shut-off valve 18 is moved back by theforce of the compression spring 104 into its blocking position in whichthe cone 102 rests on the valve seat 100, and the pressure medium isconfined in the cylinder chamber 6 without any leakage.

In order to lower the load, the control port a is connected with thetank or with a low-pressure source, while a control pressure acts on thecontrol port b to displace the main slide 12 to the left. Owing to theplay S adjusted via the spring cup 56, the main slide 12 initiallyperforms an initial stroke during which the reset spring 68 does notdevelop any effect yet. During this initial stroke the tappet 22contacts the pilot piston 24, so that the axial projection thereofforming a closure cone is raised from the pilot control seat 138. Inother words, during this initial stroke the axial displacement of themain slide 12 takes place against the force of the pilot spring 136corresponding, e.g., to a pressure of about 5 bar. As a result of theaxial displacement of the pilot piston 24, the connection between thepilot control seat 152 and the control passage 130 is graduallycontrolled open via the fine-control notches 139, so that pressuremedium may flow off from the spring chamber 105 of the shut-off valve 18via the radial bore 132, the control passage 130, the fine-controlnotches 139, the pilot control seat 138 and the control chamber 142 tothe control port a that is connected with the pressure medium tank—theshut-off valve 18 is relieved of load on its rear side. Subsequently thecone 102 may be raised from the valve seat 100 by the load pressureacting on its annular surface 154, so that the pressure medium may flowoff from the work port A via the passage 110, the axial bore 98, theanother work passage 32, and the tank passage section 50 controlled openby the annular collar 54 towards the tank port T. Concurrently thepressure medium is conducted via the metering throttle 82, the pressurecompensator 16, the connecting passage 48, and the work passage 26controlled open by the control collar 60 to the work port B and fromthere into the annular chamber 8—the hydraulic cylinder 4 retracts.

A valve assembly for load pressure-independent control of consumers isdisclosed, which includes a main valve having a directional element anda metering throttle part. The valve assembly includes a shut-off valvefor leakage-free shutting off of a work line leading to the consumer.This shut-off valve is designed with a pilot valve whereby a pressurechamber of the shut-off valve acting in the closing direction may berelieved towards a tank or a low-pressure source. The pilot valve is, inaccordance with the invention, controlled open by a main slide of themain valve.

1. Valve assembly for load pressure-independent control of consumers,including a main valve for regulating a pressure medium flow to or fromthe consumer, wherein in a pressure medium flow path between a work portand the main valve a shut-off valve is arranged which permits a pressuremedium flow to the work port and which is capable of being controlledopen in the opposite direction through the intermediary of a pilotvalve, wherein a pilot piston of the pilot valve is capable of beingraised from a pilot control seat by displacing a main slide of the mainvalve, and the valve axis of the pilot valve is arranged at a paralleldistance from the axis of the shut-off valve and about coaxially withthe axis of the main valve, and the main slide and the pilot piston arecapable of being taken into contact via a tappet.
 2. Valve assembly inaccordance with claim 1, wherein the tappet is formed on the main slide.3. Valve assembly in accordance with claim 1, wherein the tappet isformed on an insert member inserted in an end face of the main slide. 4.Valve assembly in accordance with claim 1, wherein the pilot valve andthe shut-off valve are arranged in a common valve housing that is addedon to an end face of a valve disc accommodating the main valve.
 5. Valveassembly in accordance with claim 4, wherein at least one work port isformed at the valve housing.
 6. Valve assembly in accordance with claim4, wherein the housing cartridge merges into a work passage of the valvedisc.
 7. Valve assembly in accordance with claim 4, wherein with the aidof the main valve a variable metering throttle is formed, downstreamfrom which a pressure compensator accommodated in the valve disc isarranged.
 8. Valve assembly in accordance with claim 1, wherein a springchamber of the shut-off valve is capable of being connected via thepilot valve with a low-pressure port, preferably a control portconducting the control pressure that acts on the main slide in adirection away from the pilot valve.
 9. Valve assembly in accordancewith claim 8, wherein the pilot piston includes fine-control notches forgradually opening the connection to the low-pressure port.
 10. Valveassembly in accordance with claim 1, wherein the shut-off valve isfollowed downstream by a pressure/anti-cavitation valve.
 11. Valveassembly in accordance with claim 10, wherein a port of thepressure/anti-cavitation valve is connected via a tank bore with a tankport, the tank bore encompassing a housing cartridge of the shut-offvalve as an annular chamber.
 12. Valve assembly in accordance with claim1, wherein there is associated to the main slide at least one resetspring means acting, after a predetermined initial stroke of the mainslide, contrary to the actuation force necessary for actuation of thepilot piston.
 13. Valve assembly in accordance with claim 12, whereinthe reset spring means comprises a reset spring supported on a springcup that enters into contact against a contact shoulder following theinitial stroke.